Conventionally, an image processing device such as a PC (personal computer) allows pieces of content, such as a video which is created by an image creating device (hardware) such as an accelerator and a still image which is created by a GUI (graphical user interface), to be displayed in separate windows and to be viewed simultaneously (in the following description, a window displaying a still image will be referred to as a “still image window”, and a window displaying a video will be referred to as a “video window”). As a technique for displaying pieces of content simultaneously as described above, chroma key compositing is known. Chroma key compositing is a scheme in which a specific color (color key) is set to an area where a video window is displayed on a main image which is drawn by a GUI or the like so as to make the area transparent to display the video window on the main image. However, in chroma key compositing, when a still image window overlaps over a video window and the color key is further used in the still image window in this overlapping area, a portion of the still image window where the color key is used also becomes transparent. To solve this problem, there is a technique for obtaining a correct composite image by performing a mask process (process to prevent the color key from becoming transparent) on an area where a window displaying a video and a window displaying a still image overlap (overlapping area) (for example, Patent Literature 1).